[4] In 2016, he began studying for a Doctor of Jurisprudence (JD) degree at the City University of Hong Kong (CUHK).

[15][16] In the 2015 District Council election, Cheng ran in Tanner constituency, previously held by veteran politician Desmond Lee.

Cheng formed a pro-democracy alliance with other candidates in North Point and Fortress Hill, aiming to contest pro-establishment seats in the election.

[21] In the 2020 Legislative Council election, Cheng was nominated by the Civic Party to run in the Hong Kong Island constituency.

[23] In the pro-democracy primaries in July 2020, Cheng took fourth place, behind Ted Hui, Tiffany Yuen, and Fergus Leung.

[12] On 25 July, Cheng fielded a series of questions from electoral officers, which challenged his eligibility in the upcoming election.

[31] Following his resignation from the Civic Party in December, Cheng said that he would not be able to participate in the rescheduled 2021 Hong Kong legislative election unless the political landscape changed drastically.

[32] On 6 January 2021, Cheng was among 53 members of the pro-democratic camp arrested under the national security law, specifically its provision regarding alleged subversion.

[35] On 1 April 2021, High Court judge Esther Toh explained her rationale for upholding Cheng's bail application.

While the prosecution had accused Cheng of taking part in the primaries with the ultimate aim of "removing Carrie Lam from office" and that he had expressed similar positions during a video forum that "may have been understood as calling for foreign interference", Toh understood that Cheng had renounced his political activism by stepping down from his position in the Civic Party and that he had a limited political influence at the time of the appeal against his bail, with a "very-low risk of reoffending.